In the past, the art has been developed of using an inverter to convert DC electric power to three-phase AC electric power and using that three-phase AC electric power to drive an AC electric motor. As one example of the drive method of an electric motor using this art, “field weakening control” weakening the magnetic field which a field system generates so as to lower the induced voltage generated in an electric motor at the time of high speed operation is known. Further, as another drive method, “voltage boosting control” charging and discharging a capacitor connected between a positive rail and negative rail of an inverter, that is, making the voltage between the positive rail and negative rail of the inverter fluctuate, is known. Further, to improve the energy conversion efficiency of an electric motor, the art of switching between these two control methods has been proposed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 2007-159368 and Japanese Unexamined Patent Publication (Kokai) No. 2006-311768).
In the art disclosed in Unexamined Patent Publication (Kokai) No. 2007-159368 and Japanese Unexamined Patent Publication (Kokai) No. 2006-311768, a circuit part for charging the capacitor becomes necessary separate from the circuit part for driving the electric motor. For this reason, the number of parts included in the circuit becomes greater, so this is not preferred. As opposed to this, the art of connecting a power supply between a neutral point of an electric motor and a negative rail of an inverter and utilizing the current flowing through the electric motor and inverter when the electric motor is operating so as to convert the electric power and thereby eliminating the need for a boosting circuit has been proposed (for example, see Japanese Unexamined Patent Publication (Kokai) No. 10-337047).